The present invention relates to a linear compressor, and more particularly, to a linear compressor having an oil supply structure suitable for a smooth lubricating operation, so as to prevent abrasion at a friction portion between a cylinder and a piston according to shortage of oil.
Generally, a linear compressor compresses a coolant gas by directly and reciprocally moving a piston by means of a magnet or a coil instead of a crank shaft, one example of which is as shown in FIGS. 1 and 2.
FIG. 1 is a vertical-sectional view of a general linear compressor in accordance with a conventional art, and FIG. 2 is a vertical-sectional view of an oil supply structure of the linear compressor in accordance with the conventional art.
As shown in the drawings, the general linear compressor includes a compressive unit 40 installed in a well-closed container 1 having a predetermined form in the horizontal direction for sucking, compressing and discharging a coolant, and an oil supply unit 50 fixed at an outside of the compressive unit 40 for supplying oil toward inside the compressive unit 40, especially, to the friction portion between a cylinder 3 and the piston 6 (to be described).
The compressive unit 40 includes a circular frame 1; a cover 2 fixedly installed at one side of the frame 1; a cylinder 3 fixedly installed at the center of the frame 1 in the horizontal direction; an inner lamination assembly 4A fixedly installed at the outer circumfer00ential surface of the frame 1 which supports the cylinder 3; an outer lamination assembly 4B fixedly installed at the inner circumferential surface of the frame 1, having a void with the inner lamination assembly 4A, for forming an induction magnetic field together with the inner lamination assembly 4A; a magnet assembly 5 insertedly positioned in the void between the inner lamination assembly 4A and the outer lamination assembly 4B, consisting of a magnet 5a and a magnet paddle 5b, and linearly and reciprocally moving by the induction magnetic field; a piston 6 integrally fixed at the magnet assembly 5 and linearly and reciprocally moving as being inserted into the cylinder 3 to suck and compress the coolant gas, the piston being elastically supported by the cover 2; a suction valve 7 mounted at the front end face of the piston 6; and a discharge cover 10 installed at the front end portion of the cylinder 3 and having a discharge valve assembly 11 inside thereof.
As shown in FIG. 2, oil inlet passage 1a communicated with the oil supply unit 50 is formed at the frame 1 in the hollow direction along the cylinder 3. A first circular oil groove 1b is formed on the inner circumferential surface of the cylinder 3, communicating with the oil inlet passage 1a. At least one oil through hole 3a for supplying oil to the friction portion with the piston 6 is formed at the cylinder 3 facing the first oil groove 1b. A second circular oil groove 6b is formed at the outer circumferential surface of the piston 6, communicating with the oil through hole 3a. An oil communicating path 1c at the inner circumferential surface of the cylinder 3 at the side facing the oil inlet passage 1a, communicating with the first oil groove 1b. The oil communicating path 1c is communicated with a oil circulation path 1d formed at the end portion thereof when the frame 1 is combined to the cylinder 3. An oil discharge hole 1e is formed at the lower side of the oil circulation path 1d. 
Meanwhile, the oil supply unit 50 includes an oil cylinder 21 attached on the bottom of the compressive unit 40; an oil piston 22 inserted into the oil cylinder 21 to divide the cylinder to the suction space and a discharge space, and having an oil communicating hole 22a for communicating the suction space and the discharge space; a first and a second oil springs 23A and 23B for elastically supporting the both ends of the oil piston 22 against the oil cylinder 21; a suction cover 24 and a discharge cover 25 for supporting each other end of the first and the second oil springs 23A and 23B and fixing the both ends of the oil cylinder 21 to the compressive unit 40; an oil suction valve 26 supported by the first oil spring 23A at the outlet side of the oil piston 22; and an oil discharge valve 27 installed at the outlet side, of the oil cylinder 21.
Reference numeral 4b denotes a coil assembly, 6a denotes a coolant passage, 8 denotes a main spring and 9 denotes a suction pipe.
The operation of the linear compressor constructed as described above will now be explained.
First, when an induction magnetic field is generated as current is applied to a stator of a linear motor consisting of the inner lamination assembly 4A and the outer lamination assembly 4B, the magnet assembly 5, a rotor in the void between the inner lamination assembly 4A and the outer lamination assembly 4B linearly reciprocates owing to the induction magnetic field, so that the piston 6 moves reciprocally in the cylinder 3. Due to the piston""s reciprocal movement in the cylinder 3, the coolant gas introduced into the well-closed container 30 is sucked into the compressed space xe2x80x98Cxe2x80x99 of the cylinder through the coolant passage 6a formed at the center of the piston 6. The compressed gas opens the discharge valve assembly 11 to be discharged to the outside from the discharge cover 10. These processes are repeatedly performed.
In the above process, the oil supply unit 50 vibrates along with the compressive unit 40, pumping the oil filling the bottom of the well-closed container 30 to supply it to the compressive unit 40. The oil pumped from the oil supply unit 50 is induced to the first oil groove 1b through the oil inlet passage 1a. 
The oil induced to the first oil groove 1b fills the second oil groove 6b through the through hole 3a, cooling the cylinder 3, and lubricates the friction portion between the cylinder 3 and the piston 6.
Thereafter, after lubricating the friction portion between the cylinder 3 and the piston 6, the oil is induced to the other first oil groove 1b through the other oil through hole 3a, cooling the discharge cover 10 along the oil communicating path 1c and the oil circulation path 1d together with the oil that filled the first oil groove 1b, and returns to the well-closed container 30 through the oil discharge hole 1e. 
However, the linear compressor of the conventional art as described above has problems in that since the oil inlet passage 1a of the frame and the oil through hole 3a of the cylinder 3 are linearly formed to the compressive unit 40, when the compressor is stopped, the oil filling the first oil groove 1b of the frame 1 and the second oil groove 6b of the piston 6 flows out through the oil of inlet passage 1a, being taken out toward the oil supply unit 50 and the well-closed container 30, resulting in that when the compressor is actuated again, friction occurs at the friction portion between the cylinder 3 and the piston 6 due to the oil shortage until the oil is re-supplied to the friction portion, and due to the friction, the piston or the cylinder may be abraded.
Accordingly, in order to overcome the problems of the conventional linear compressor, an object of the present invention is to provide a linear compressor having an oil backflow prevention device by which oil always exists at the friction portion between a cylinder and a piston so that even when a compressor is stopped and re-started, the friction portion is smoothly lubricated.
In order to accomplish the above object, there is provided a linear compressor including a well-closed container filled with oil at its bottom, a compressive unit having a frame of a hollow inside the well-closed container to which a cylinder inserted, a piston inserted into the hollow of the cylinder, for linearly moving upward and downward by driving of a motor installed at the cylinder, and a discharge cover for covering one end of the cylinder, and an oil supply unit mounted at the lower side of the compressive unit, for pumping oil, wherein the oil supplied from the oil supply unit to the compressive unit exists within the compressor unit in either case that the compressor is operated or stopped.